Based on the daily reanalysis data from May to August in 1998,it is found that the transient and quasi permanent structures are different from each other for western Pacific subtropical high(WPSTH).The downdraft in WPSTH center in daily data is mainly caused by the latent heat release in its northern monsoon rainbelt which gives rise to ascending motion there and descending motion in the center,therefrom explaining the mechanism of WPSTH formation and variation.The lagging correlation analysis between convective precipitation rate and area index, and westward extension index of WPSTH indicates that the obvious interaction exists between the westward extension of WPSTH and its surrounding rainbelt.